Selection in fungi

ABSTRACT

The present invention relates to methods for constructing a recombinant fungal host cell comprising one or more copies of a polynucleotide construct integrated in its genome, said method comprising transforming a fungal host cell with an integrative polynucleotide construct comprising a first polynucleotide encoding a selectable marker, wherein the first polynucleotide, a 5′ untranslated region thereof and/or a riboswitch operably linked therewith comprises a spliceosomal intron which has 5 nucleotides or less between its branch site and its acceptor site; and a second polynucleotide encoding a polypeptide of interest; as well as suitable polynucleotide constructs, resulting fungal host cells and methods of manufacture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 14/403,355 filed May 29, 2013, now U.S. Pat. No. 9,487,767, which is a 35 U.S.C. § 371 national application of PCT/EP2013/061052 filed on May 29, 2013, which claims priority or the benefit under 35 U.S.C. § 119 of European Application No. 12170260.9 filed on May 31, 2012 and U.S. Provisional Application No. 61/656,170 filed on Jun. 6, 2012, the contents of which are fully incorporated herein by reference.

REFERENCE TO A SEQUENCE LISTING

This application contains a Sequence Listing in computer readable form, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to methods for constructing a recombinant fungal host cell comprising one or more copies of a polynucleotide construct integrated in its genome, said method comprising transforming a fungal host cell with an integrative polynucleotide construct comprising a first polynucleotide encoding a selectable marker, wherein the first polynucleotide, a 5′ untranslated region thereof and/or a riboswitch operably linked therewith comprises a spliceosomal intron which has 5 nucleotides or less between its branch site and its acceptor site; and a second polynucleotide encoding a polypeptide of interest; as well as suitable polynucleotide constructs, resulting fungal host cells and methods of manufacture.

BACKGROUND OF THE INVENTION

It is desirable for the biotech industry to provide microbial strains devoid of antibiotic resistance markers comprising several chromosomally integrated copies of a gene of interest, for the industrial high yield production of polypeptides. Antibiotic marker genes have traditionally been used as a means to select for strains carrying multiple copies of both the marker gene and an accompanying expression cassette coding for a polypeptide of industrial interest. Amplification of the expression cassette by increasing the copy number in a microbiological production strain was desirable because there is very often a direct correlation between the number of copies and the final product yields. Amplification methods using antibiotic selection markers have been used extensively in many host strains over the past 20 years and have proven to be a very efficient way to develop high yielding production strains in a relatively short time, irrespective of the expression level of the individual expression cassettes.

It has previously been shown in Bacillus that a galactose epimerase-encoding gene expressed from a crippled low-level promoter could be used as a selection marker for site-specific genomic integration of tandemly amplified copies of a product-gene (WO 2001/90393; Novozymes A/S). However, no similar systems have been described for fungal host cells.

A riboswitch is part of an mRNA molecule that can bind directly to a small target molecule without a protein being involved. Binding of the small target molecule will affect the translation of the mRNA [Nudler E, Mironov A S (2004). “The riboswitch control of bacterial metabolism”. Trends Biochem Sci 29 (1): 11-7; Vitreschak A G, Rodionov D A, Mironov A A, Gelfand M S (2004). “Riboswitches: the oldest mechanism for the regulation of gene expression?”. Trends Genet 20 (1): 44-50; Tucker B J, Breaker R R (2005). “Riboswitches as versatile gene control elements”. Curr Opin Struct Biol 15 (3): 342-8; Batey R T (2006). “Structures of regulatory elements in mRNAs”. Curr Opin Struct Biol 16 (3): 299-306].

In the filamentous fungal cell Aspergillus oryzae expression of the thiA and nmtA genes are regulated by riboswitches that bind thiamine pyrophosphate (TPP) and controls alternative splicing to conditionally produce an upstream Open Reading Frame (ORF), thereby affecting the expression of the downstream gene(s). The thiA riboswitch of A. oryzae contains a nuclear pre-mRNA intron, a spliceosomal intron, which is involved in facilitating the alternative splicing.

Another filamentous fungal riboswitch has been found in Aspergillus nidulans, where the agaA gene is regulated by mRNA arginine-binding, thereby facilitating alternative splicing [Borsuk, P. et al. (2007). “L-Arginine influences the structure and function of arginase mRNA in Aspergillus nidulans”. Biol Chem. 388: 135-144].

In order to comply with the current demand for recombinant fungal production host strains devoid of antibiotic markers, we have looked for possible alternatives to produce multi-copy host strains.

SUMMARY OF THE INVENTION

The inventors found that by varying the number of nucleotides between the branch site and the acceptor site in a spliceosomal intron located in the 5′ untranslated region of a downstream gene, the splicing efficiency could be adjusted to provide a surprisingly large expression range for the downstream gene. The number of nucleotides in the wildtype intron was 6 and the inventors successively removed each nucleotide until the branch site and the acceptor site actually overlapped with one nucleotide. This library of intron variant provided an astonishing range of expression levels of a downstream gene, from normal expression to an extremely low level of expression, as the examples below demonstrate. The inventors expect this to be the case also for spliceosomal introns located within the coding sequence of a gene.

Low expression levels are particularly interesting in the context of selectable markers comprised in polynucleotide constructs to be integrated into the genome of fungal host cells, because they make it possible to select for transformed cells, wherein the polynucleotide constructs have been integrated into the genome in multiple tandemly amplified copies. This selection is made possible because those cells having many copies of a selectable marker expressed at sufficiently low levels will have a growth advantage over cells with fewer copies, when cultivated under selective pressure. The expression cassette in the integrative construct will be amplified along with the selection marker, thereby ensuring a higher product yield.

Further, the particular spliceosomal intron investigated by the inventors herein was located within a so-called riboswitch, the thiA riboswitch of Aspergillus oryzae, where it is normally involved in the alternative splicing of the pre-mRNA of that gene, thereby regulating its expression level depending on whether or not there is enough thiamine (or rather, thiamine-pyrophosphate, TPP) for the cell's needs. Notwithstanding the surprisingly wide range of expression levels displayed by the intron variants created herein, the inventors found that addition of TPP to host cells comprising the intron variants of the invention suppressed the expression level even further via the action of the thiA riboswitch—in some cases below detection level.

Accordingly, in a first aspect the present invention relates to methods for constructing a recombinant fungal host cell comprising one or more copies of a polynucleotide construct integrated in its genome, said methods comprising:

-   -   a) providing a fungal host cell transformed with an integrative         polynucleotide construct, said construct comprising a first         polynucleotide encoding a selectable marker, wherein the first         polynucleotide, a 5′ untranslated region thereof and/or a         riboswitch operably linked therewith comprises a spliceosomal         intron which has 5 nucleotides or less between its branch site         and its acceptor site; and a second polynucleotide encoding a         polypeptide of interest;     -   b) cultivating the transformed fungal host cell under conditions         conducive for expressing the selectable marker; and     -   c) isolating a recombinant fungal host cell comprising one or         more copies of the polynucleotide construct integrated in its         genome.

In a second aspect the invention relates to polynucleotide constructs suitable for transformation into a fungal host cell and integration into the genome of said cell, said construct comprising:

-   -   a) a first polynucleotide encoding a selectable marker, wherein         the first polynucleotide, a 5′ untranslated region thereof         and/or a riboswitch operably linked therewith comprises a         spliceosomal intron which has 5 nucleotides or less between its         branch site and its acceptor site; and     -   b) a second polynucleotide encoding a polypeptide of interest.

In a third aspect the invention relates to recombinant fungal host cells comprising one or more copies of a polynucleotide construct integrated in its genome, said construct comprising:

-   -   a) a first polynucleotide encoding a selectable marker, wherein         the first polynucleotide, a 5′ untranslated region thereof         and/or a riboswitch operably linked therewith comprises a         spliceosomal intron which has 5 nucleotides or less between its         branch site and its acceptor site; and     -   b) a second polynucleotide encoding a polypeptide of interest.

In a final aspect, the invention relates to methods of producing a polypeptide of interest, comprising:

-   -   a) cultivating a recombinant fungal host cell according to the         third aspect; and optionally     -   b) recovering the polypeptide.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic overview of plasmid pCOIs1086 of the examples below.

FIG. 2 shows a schematic overview of plasmid pCOIs1124 of the examples below.

FIG. 3 shows a schematic overview of plasmid pCOIs1118 of the examples below.

FIG. 4 shows a schematic overview of plasmid pCOIs1126 of the examples below.

FIG. 5 shows a general illustration of spliceosomal intron RNA processing as well as eight different construct made with different distances between the intron branch site “ctaac” (shaded) and the intron acceptor site “tag” (shaded) in the thiA riboswitch intron. The number of bases between the two sites were varied between 0-6 bp and one construct had the acceptor site overlapping with and forming a part of the branch site, resulting in the sequence “ctaacag”, where the branch site is “ctaac” and the acceptor site “cag”. The latter variant was named “Intron(−1)”. The others were named “Intron(0)”, “Intron(1)”, “Intron(2), “Intron(3)”, “Intron(4)”, “Intron(5)” and finally “Intron(6)” which is the wild type sequence (FIG. 5), as shown in the table below:

ctaacagaTGATAGTCATTG Intron (6) SEQ ID NO: 33 ctaactgagatagTTGATTG Intron (5) SEQ ID NO: 34 ctaactgaatagTCTGATTG Intron (4) SEQ ID NO: 35 ctaactgatagTGATGATTG Intron (3) SEQ ID NO: 36 ctaacgatagTTGATGATTG Intron (2) SEQ ID NO: 37 ctaacatagTCTGATGATTG Intron (1) SEQ ID NO: 38 ctaactagTCATGATGATTG Intron (0) SEQ ID NO: 39 ctaacagatgatccTCATTG Intron (-1) SEQ ID NO: 40

FIG. 6 shows a picture of the Southern blot in example 4 below; Lane 1 is Lambda marker DNA digested with BstEII; Lane 2 is a strain transformed with pCOIs1175 and Lane 3 is the background strain.

DEFINITIONS

cDNA: The term “cDNA” means a DNA molecule that can be prepared by reverse transcription from a mature, spliced, mRNA molecule obtained from a eukaryotic or prokaryotic cell. cDNA lacks intron sequences that may be present in the corresponding genomic DNA. The initial, primary RNA transcript is a precursor to mRNA that is processed through a series of steps, including splicing, before appearing as mature spliced mRNA.

Coding sequence: The term “coding sequence” means a polynucleotide, which directly specifies the amino acid sequence of a polypeptide. The boundaries of the coding sequence are generally determined by an open reading frame, which begins with a start codon such as ATG, GTG, or TTG and ends with a stop codon such as TAA, TAG, or TGA. The coding sequence may be a genomic DNA, cDNA, synthetic DNA, or a combination thereof.

Control sequences: The term “control sequences” means nucleic acid sequences necessary for expression of a polynucleotide encoding a mature polypeptide of the present invention. Each control sequence may be native (i.e., from the same gene) or foreign (i.e., from a different gene) to the polynucleotide encoding the polypeptide or native or foreign to each other. Such control sequences include, but are not limited to, a leader, polyadenylation sequence, propeptide sequence, promoter, signal peptide sequence, and transcription terminator. At a minimum, the control sequences include a promoter, and transcriptional and translational stop signals. The control sequences may be provided with linkers for the purpose of introducing specific restriction sites facilitating ligation of the control sequences with the coding region of the polynucleotide encoding a polypeptide.

Expression: The term “expression” includes any step involved in the production of a polypeptide including, but not limited to, transcription, post-transcriptional modification, translation, post-translational modification, and secretion.

Expression vector: The term “expression vector” means a linear or circular DNA molecule that comprises a polynucleotide encoding a polypeptide and is operably linked to control sequences that provide for its expression.

Host cell: The term “host cell” means any cell type that is susceptible to transformation, transfection, transduction, or the like with a nucleic acid construct or expression vector comprising a polynucleotide of the present invention. The term “host cell” encompasses any progeny of a parent cell that is not identical to the parent cell due to mutations that occur during replication.

Isolated: The term “isolated” means a substance in a form or environment that does not occur in nature. Non-limiting examples of isolated substances include (1) any non-naturally occurring substance, (2) any substance including, but not limited to, any enzyme, variant, nucleic acid, protein, peptide or cofactor, that is at least partially removed from one or more or all of the naturally occurring constituents with which it is associated in nature; (3) any substance modified by the hand of man relative to that substance found in nature; or (4) any substance modified by increasing the amount of the substance relative to other components with which it is naturally associated (e.g., multiple copies of a gene encoding the substance; use of a stronger promoter than the promoter naturally associated with the gene encoding the substance). An isolated substance may be present in a fermentation broth sample.

Nucleic acid construct: The term “nucleic acid construct” means a nucleic acid molecule, either single- or double-stranded, which is isolated from a naturally occurring gene or is modified to contain segments of nucleic acids in a manner that would not otherwise exist in nature or which is synthetic, which comprises one or more control sequences.

Operably linked: The term “operably linked” means a configuration in which a control sequence is placed at an appropriate position relative to the coding sequence of a polynucleotide such that the control sequence directs expression of the coding sequence.

Sequence identity: The relatedness between two amino acid sequences or between two nucleotide sequences is described by the parameter “sequence identity”.

For purposes of the present invention, the sequence identity between two amino acid sequences is determined using the Needleman-Wunsch algorithm (Needleman and Wunsch, 1970, J. Mol. Biol. 48: 443-453) as implemented in the Needle program of the EMBOSS package (EMBOSS: The European Molecular Biology Open Software Suite, Rice et al., 2000, Trends Genet. 16: 276-277), preferably version 5.0.0 or later. The parameters used are gap open penalty of 10, gap extension penalty of 0.5, and the EBLOSUM62 (EMBOSS version of BLOSUM62) substitution matrix. The output of Needle labeled “longest identity” (obtained using the −nobrief option) is used as the percent identity and is calculated as follows: (Identical Residues×100)/(Length of Alignment−Total Number of Gaps in Alignment)

For purposes of the present invention, the sequence identity between two deoxyribonucleotide sequences is determined using the Needleman-Wunsch algorithm (Needleman and Wunsch, 1970, supra) as implemented in the Needle program of the EMBOSS package (EMBOSS: The European Molecular Biology Open Software Suite, Rice et al., 2000, supra), preferably version 5.0.0 or later. The parameters used are gap open penalty of 10, gap extension penalty of 0.5, and the EDNAFULL (EMBOSS version of NCBI NUC4.4) substitution matrix. The output of Needle labeled “longest identity” (obtained using the −nobrief option) is used as the percent identity and is calculated as follows: (Identical Deoxyribonucleotides×100)/(Length of Alignment Total Number of Gaps in Alignment)

Subsequence: The term “subsequence” means a polynucleotide having one or more (e.g., several) nucleotides absent from the 5′ and/or 3′ end of a mature polypeptide coding sequence; wherein the subsequence encodes a fragment having enzyme activity.

Spliceosomal intron: The term “spliceosomal intron” is another term for nuclear pre-mRNA introns that are characterized by specific intron sequences located at the boundaries between introns and exons. These sequences are recognized by spliceosomal RNA molecules when the splicing reactions are initiated. In addition, they contain a branch site, a particular nucleotide sequence near the 3′ end of the intron that becomes covalently linked to the 5′ end, the acceptor site, of the intron during the splicing process, generating a branched intron. Apart from these three short conserved elements, nuclear pre-mRNA intron sequences are highly variable.

Intron branch site: In the present context, an intron branch site is defined by the nucleotide sequence “CTRAY”, where R=A or G and Y=C or T, in particular as “CTAAC”.

Intron acceptor site: An intron acceptor site is often characterized by the two nucleotides AG, but the nucleotide directly upstream may well influence the splicing efficiency, so in the present context, an intron acceptor site is defined as: XAG, wherein X=G or A or T or C.

DETAILED DESCRIPTION OF THE INVENTION

A method for constructing a recombinant fungal host cell comprising one or more copies of a polynucleotide construct integrated in its genome, said method comprising:

a) providing a fungal host cell transformed with an integrative polynucleotide construct, said construct comprising a first polynucleotide encoding a selectable marker, wherein the first polynucleotide, a 5′ untranslated region thereof and/or a riboswitch operably linked therewith comprises a spliceosomal intron which has 5 nucleotides or less between its branch site and its acceptor site; and a second polynucleotide encoding a polypeptide of interest; b) cultivating the transformed fungal host cell under conditions conducive for expressing the selectable marker; and c) isolating a recombinant fungal host cell comprising one or more copies of the polynucleotide construct integrated in its genome; preferably two or more copies; even more preferably three or more copies and most preferably four or more copies.

In a preferred embodiment of the first aspect, the fungal host cell in step (a) has a growth deficiency and the integrative polynucleotide construct complements said growth deficiency when integrated into the genome of the host cell. This allows an optional easy second step between steps (a) and (b) of selecting of a fungal cell, wherein genomic integration of the polynucleotide construct in at least one copy has been successful.

With respect to suitable growth deficiencies, it is preferred that the fungal host cell in step (a) lacks a functional nitrate reductase or nitrite reductase and that the integrative polynucleotide construct comprises a complementing gene encoding a functional nitrate reductase, such as, niaD, or a gene encoding a functional nitrite reductase, such as, niiA, respectively; or that the fungal host cell in step (a) lacks a functional enolase and the integrative polynucleotide construct comprises a complementing gene encoding a functional enolase, such as, acuN.

Other suitable deficiencies relate to required functionalities of a fungal cell grown on a gluconeogenic carbon source. For example, it is preferred that the fungal host of step (a) has an inactive acuK or acuM gene and that the integrative polynucleotide construct comprises a complementing functional acuK or acuM gene, respectively [Hynes M. et al. Transcriptional Control of Gluconeogenesis in Aspergillus nidulans, Genetics 176: 139-150 (May 2007)].

Preferably, the integrative polynucleotide construct is randomly integrated in the genome by non-homologous recombination after transformation.

However, often a greater degree of control is preferred, where the integrative polynucleotide construct in step (a) is flanked on one or both side(s) by a homology box of sufficient size and sequence homology to a specific locus in the fungal host cell genome to enable site-specific integration of the integrative polynucleotide construct into said genome by homologous recombination after transformation. Conveniently, the specific locus in the fungal host is the same gene which has been inactivated, for example by partial deletion, to create a growth deficiency. The homology box(es) of the polynucleotide construct can then be either a full copy of the same gene or parts thereof, so that the successful site-specific integration of at least one copy of the polynucleotide construct through homologous recombination between the homology box(es) and the genomic inactivated gene results in the restoration of a functional gene in the genome or a replacement with a functional version of the same gene.

Preferably, the spliceosomal intron has 4 nucleotides or less between its branch site and its acceptor site; preferably 3 nucleotides or less; more preferably 2 nucleotides or less; even more preferably 1 nucleotide; still more preferably the spliceosomal intron has 0 nucleotides between its branch site and its acceptor site and most preferably the branch site and the acceptor site of spliceosomal intron overlap by at least one nucleotide. Alternatively, it is preferred that the spliceosomal intron comprises a nucleotide sequence selected from the group of intron nucleotide sequences consisting of SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40.

We have already mentioned, that the inventors carried out the method of the first aspect using a spliceosomal intron of the thiA riboswitch from A. oryzae and showed that this enabled an even greater degree of expression level control of the downstream gene(s). Accordingly, in a preferred embodiment of the method of the first aspect, the first polynucleotide of the integrative polynucleotide construct has a riboswitch operably linked therewith; preferably the riboswitch is derived from the thiA or nmtA genes in Aspergillus oryzae.

Preferably, the first polynucleotide of the integrative polynucleotide construct encodes the selectable marker orotidine-5′ phosphate decarboxylase or PyrG.

In a preferred embodiment of the invention, the polypeptide of interest encoded by the second polynucleotide of the integrative polynucleotide construct comprises an enzyme; preferably a hydrolase, isomerase, ligase, lyase, oxidoreductase, or transferase, e.g., an aminopeptidase, amylase, carbohydrase, carboxypeptidase, catalase, cellobiohydrolase, cellulase, chitinase, cutinase, cyclodextrin glycosyltransferase, deoxyribonuclease, endoglucanase, esterase, alpha-galactosidase, beta-galactosidase, glucoamylase, alpha-glucosidase, beta-glucosidase, invertase, laccase, lipase, mannosidase, mutanase, oxidase, pectinolytic enzyme, peroxidase, phytase, polyphenoloxidase, proteolytic enzyme, ribonuclease, transglutaminase, xylanase, or beta-xylosidase.

Nucleic Acid Constructs

A second aspect of the invention relates to a polynucleotide construct suitable for transformation into a fungal host cell and integration into the genome of said cell, said construct comprising: a first polynucleotide encoding a selectable marker, wherein the first polynucleotide, a 5′ untranslated region thereof and/or a riboswitch operably linked therewith comprises a spliceosomal intron which has 5 nucleotides or less between its branch site and its acceptor site; and a second polynucleotide encoding a polypeptide of interest.

In a preferred embodiment of the second aspect, the polynucleotide construct comprises a gene encoding a functional nitrate reductase, such as, niaD, a gene encoding a functional nitrite reductase, such as, niiA, or a gene encoding a functional enolase, such as, acuN.

In a preferred embodiment of the second aspect, the polynucleotide construct is flanked on one or both side(s) by a homology box of sufficient size and sequence homology to a specific locus in the fungal host cell genome to enable site-specific integration of the integrative polynucleotide construct into said genome by homologous recombination after transformation.

Preferably, the spliceosomal intron has 4 nucleotides or less between its branch site and its acceptor site; preferably 3 nucleotides or less; more preferably 2 nucleotides or less; even more preferably 1 nucleotide; still more preferably the spliceosomal intron has 0 nucleotides between its branch site and its acceptor site and most preferably the branch site and the acceptor site of spliceosomal intron overlap by at least one nucleotide. Or alternatively, the spliceosomal intron comprises a nucleotide sequence selected from the group of intron nucleotide sequences consisting of SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40.

In a preferred embodiment the first polynucleotide, located in the integrative polynucleotide construct of the invention, has a riboswitch operably linked therewith; preferably the riboswitch is derived from the thiA or nmtA genes in Aspergillus oryzae.

Preferably, the first polynucleotide of the integrative polynucleotide construct encodes the selectable marker orotidine-5′ phosphate decarboxylase or PyrG.

It is preferable, that the polypeptide of interest encoded by the second polynucleotide of the integrative polynucleotide construct comprises an enzyme; preferably a hydrolase, isomerase, ligase, lyase, oxidoreductase, or transferase, e.g., an aminopeptidase, amylase, carbohydrase, carboxypeptidase, catalase, cellobiohydrolase, cellulase, chitinase, cutinase, cyclodextrin glycosyltransferase, deoxyribonuclease, endoglucanase, esterase, alpha-galactosidase, beta-galactosidase, glucoamylase, alpha-glucosidase, beta-glucosidase, invertase, laccase, lipase, mannosidase, mutanase, oxidase, pectinolytic enzyme, peroxidase, phytase, polyphenoloxidase, proteolytic enzyme, ribonuclease, transglutaminase, xylanase, or beta-xylosidase.

The present invention also relates to nucleic acid constructs comprising a first and/or second polynucleotide of the present invention operably linked to one or more control sequences that direct the expression of the coding sequence in a suitable host cell under conditions compatible with the control sequences.

A polynucleotide may be manipulated in a variety of ways to provide for expression of the polypeptide. Manipulation of the polynucleotide prior to its insertion into a vector may be desirable or necessary depending on the expression vector. The techniques for modifying polynucleotides utilizing recombinant DNA methods are well known in the art.

The control sequence may be a promoter, a polynucleotide that is recognized by a host cell for expression of a polynucleotide encoding a polypeptide of the present invention. The promoter contains transcriptional control sequences that mediate the expression of the polypeptide. The promoter may be any polynucleotide that shows transcriptional activity in the host cell including mutant, truncated, and hybrid promoters, and may be obtained from genes encoding extracellular or intracellular polypeptides either homologous or heterologous to the host cell.

Examples of suitable promoters for directing transcription of the nucleic acid constructs of the present invention in a filamentous fungal host cell are promoters obtained from the genes for Aspergillus nidulans acetamidase, Aspergillus niger neutral alpha-amylase, Aspergillus niger acid stable alpha-amylase, Aspergillus niger or Aspergillus awamori glucoamylase (glaA), Aspergillus oryzae TAKA amylase, Aspergillus oryzae alkaline protease, Aspergillus oryzae triose phosphate isomerase, Fusarium oxysporum trypsin-like protease (WO 96/00787), Fusarium venenatum amyloglucosidase (WO 00/56900), Fusarium venenatum Dania (WO 00/56900), Fusarium venenatum Quinn (WO 00/56900), Rhizomucor miehei lipase, Rhizomucor miehei aspartic proteinase, Trichoderma reesei beta-glucosidase, Trichoderma reesei cellobiohydrolase I, Trichoderma reesei cellobiohydrolase II, Trichoderma reesei endoglucanase I, Trichoderma reesei endoglucanase II, Trichoderma reesei endoglucanase III, Trichoderma reesei endoglucanase IV, Trichoderma reesei endoglucanase V, Trichoderma reesei xylanase I, Trichoderma reesei xylanase II, Trichoderma reesei beta-xylosidase, as well as the NA2-tpi promoter (a modified promoter from an Aspergillus neutral alpha-amylase gene in which the untranslated leader has been replaced by an untranslated leader from an Aspergillus triose phosphate isomerase gene; non-limiting examples include modified promoters from an Aspergillus niger neutral alpha-amylase gene in which the untranslated leader has been replaced by an untranslated leader from an Aspergillus nidulans or Aspergillus oryzae triose phosphate isomerase gene); and mutant, truncated, and hybrid promoters thereof.

In a yeast host, useful promoters are obtained from the genes for Saccharomyces cerevisiae enolase (ENO-1), Saccharomyces cerevisiae galactokinase (GAL1), Saccharomyces cerevisiae alcohol dehydrogenase/glyceraldehyde-3-phosphate dehydrogenase (ADH1, ADH2/GAP), Saccharomyces cerevisiae triose phosphate isomerase (TPI), Saccharomyces cerevisiae metallothionein (CUP1), and Saccharomyces cerevisiae 3-phosphoglycerate kinase. Other useful promoters for yeast host cells are described by Romanos et al., 1992, Yeast 8: 423-488.

The control sequence may also be a transcription terminator, which is recognized by a host cell to terminate transcription. The terminator is operably linked to the 3′-terminus of the polynucleotide encoding the polypeptide. Any terminator that is functional in the host cell may be used in the present invention.

Preferred terminators for filamentous fungal host cells are obtained from the genes for Aspergillus nidulans anthranilate synthase, Aspergillus niger glucoamylase, Aspergillus niger alpha-glucosidase, Aspergillus oryzae TAKA amylase, and Fusarium oxysporum trypsin-like protease.

Preferred terminators for yeast host cells are obtained from the genes for Saccharomyces cerevisiae enolase, Saccharomyces cerevisiae cytochrome C (CYC1), and Saccharomyces cerevisiae glyceraldehyde-3-phosphate dehydrogenase. Other useful terminators for yeast host cells are described by Romanos et al., 1992, supra.

The control sequence may also be an mRNA stabilizer region downstream of a promoter and upstream of the coding sequence of a gene which increases expression of the gene.

Examples of suitable mRNA stabilizer regions are obtained from a Bacillus thuringiensis cryIIIA gene (WO 94/25612) and a Bacillus subtilis SP82 gene (Hue et al., 1995, Journal of Bacteriology 177: 3465-3471).

The control sequence may also be a leader, a nontranslated region of an mRNA that is important for translation by the host cell. The leader is operably linked to the 5′-terminus of the polynucleotide encoding the polypeptide. Any leader that is functional in the host cell may be used.

Preferred leaders for filamentous fungal host cells are obtained from the genes for Aspergillus oryzae TAKA amylase and Aspergillus nidulans triose phosphate isomerase.

Suitable leaders for yeast host cells are obtained from the genes for Saccharomyces cerevisiae enolase (ENO-1), Saccharomyces cerevisiae 3-phosphoglycerate kinase, Saccharomyces cerevisiae alpha-factor, and Saccharomyces cerevisiae alcohol dehydrogenase/glyceraldehyde-3-phosphate dehydrogenase (ADH2/GAP).

The control sequence may also be a polyadenylation sequence, a sequence operably linked to the 3′-terminus of the polynucleotide and, when transcribed, is recognized by the host cell as a signal to add polyadenosine residues to transcribed mRNA. Any polyadenylation sequence that is functional in the host cell may be used.

Preferred polyadenylation sequences for filamentous fungal host cells are obtained from the genes for Aspergillus nidulans anthranilate synthase, Aspergillus niger glucoamylase, Aspergillus niger alpha-glucosidase Aspergillus oryzae TAKA amylase, and Fusarium oxysporum trypsin-like protease.

Useful polyadenylation sequences for yeast host cells are described by Guo and Sherman, 1995, Mol. Cellular Biol. 15: 5983-5990.

The control sequence may also be a signal peptide coding region that encodes a signal peptide linked to the N-terminus of a polypeptide and directs the polypeptide into the cell's secretory pathway. The 5′-end of the coding sequence of the polynucleotide may inherently contain a signal peptide coding sequence naturally linked in translation reading frame with the segment of the coding sequence that encodes the polypeptide. Alternatively, the 5′-end of the coding sequence may contain a signal peptide coding sequence that is foreign to the coding sequence. A foreign signal peptide coding sequence may be required where the coding sequence does not naturally contain a signal peptide coding sequence. Alternatively, a foreign signal peptide coding sequence may simply replace the natural signal peptide coding sequence in order to enhance secretion of the polypeptide. However, any signal peptide coding sequence that directs the expressed polypeptide into the secretory pathway of a host cell may be used.

Effective signal peptide coding sequences for filamentous fungal host cells are the signal peptide coding sequences obtained from the genes for Aspergillus niger neutral amylase, Aspergillus niger glucoamylase, Aspergillus oryzae TAKA amylase, Humicola insolens cellulase, Humicola insolens endoglucanase V, Humicola lanuginosa lipase, and Rhizomucor miehei aspartic proteinase.

Useful signal peptides for yeast host cells are obtained from the genes for Saccharomyces cerevisiae alpha-factor and Saccharomyces cerevisiae invertase. Other useful signal peptide coding sequences are described by Romanos et al., 1992, supra.

The control sequence may also be a propeptide coding sequence that encodes a propeptide positioned at the N-terminus of a polypeptide. The resultant polypeptide is known as a proenzyme or propolypeptide (or a zymogen in some cases). A propolypeptide is generally inactive and can be converted to an active polypeptide by catalytic or autocatalytic cleavage of the propeptide from the propolypeptide. The propeptide coding sequence may be obtained from the genes for Bacillus subtilis alkaline protease (aprE), Bacillus subtilis neutral protease (nprT), Myceliophthora thermophila laccase (WO 95/33836), Rhizomucor miehei aspartic proteinase, and Saccharomyces cerevisiae alpha-factor.

Where both signal peptide and propeptide sequences are present, the propeptide sequence is positioned next to the N-terminus of a polypeptide and the signal peptide sequence is positioned next to the N-terminus of the propeptide sequence.

It may also be desirable to add regulatory sequences that regulate expression of the polypeptide relative to the growth of the host cell. Examples of regulatory systems are those that cause expression of the gene to be turned on or off in response to a chemical or physical stimulus, including the presence of a regulatory compound. Regulatory systems in prokaryotic systems include the lac, tac, and trp operator systems. In yeast, the ADH2 system or GAL1 system may be used. In filamentous fungi, the Aspergillus niger glucoamylase promoter, Aspergillus oryzae TAKA alpha-amylase promoter, and Aspergillus oryzae glucoamylase promoter may be used. Other examples of regulatory sequences are those that allow for gene amplification. In eukaryotic systems, these regulatory sequences include the dihydrofolate reductase gene that is amplified in the presence of methotrexate, and the metallothionein genes that are amplified with heavy metals. In these cases, the polynucleotide encoding the polypeptide would be operably linked with the regulatory sequence.

Expression Vectors

The present invention also relates to recombinant expression vectors comprising a polynucleotide of the present invention, a promoter, and transcriptional and translational stop signals. The various nucleotide and control sequences may be joined together to produce a recombinant expression vector that may include one or more convenient restriction sites to allow for insertion or substitution of the polynucleotide encoding the polypeptide at such sites. Alternatively, the polynucleotide may be expressed by inserting the polynucleotide or a nucleic acid construct comprising the polynucleotide into an appropriate vector for expression. In creating the expression vector, the coding sequence is located in the vector so that the coding sequence is operably linked with the appropriate control sequences for expression.

The recombinant expression vector may be any vector (e.g., a plasmid or virus) that can be conveniently subjected to recombinant DNA procedures and can bring about expression of the polynucleotide. The choice of the vector will typically depend on the compatibility of the vector with the host cell into which the vector is to be introduced. The vector may be a linear or closed circular plasmid.

The vector may be an autonomously replicating vector, i.e., a vector that exists as an extrachromosomal entity, the replication of which is independent of chromosomal replication, e.g., a plasmid, an extrachromosomal element, a minichromosome, or an artificial chromosome. The vector may contain any means for assuring self-replication. Alternatively, the vector may be one that, when introduced into the host cell, is integrated into the genome and replicated together with the chromosome(s) into which it has been integrated. Furthermore, a single vector or plasmid or two or more vectors or plasmids that together contain the total DNA to be introduced into the genome of the host cell, or a transposon, may be used.

The vector preferably contains one or more selectable markers that permit easy selection of transformed, transfected, transduced, or the like cells. A selectable marker is a gene the product of which provides for biocide or viral resistance, resistance to heavy metals, prototrophy to auxotrophs, and the like.

Selectable markers for use in a filamentous fungal host cell include, but are not limited to, amdS (acetamidase), argB (ornithine carbamoyltransferase), bar (phosphinothricin acetyltransferase), hph (hygromycin phosphotransferase), niaD (nitrate reductase), pyrG (orotidine-5′-phosphate decarboxylase), sC (sulfate adenyltransferase), and trpC (anthranilate synthase), as well as equivalents thereof. Preferred for use in an Aspergillus cell are Aspergillus nidulans or Aspergillus olyzae amdS and pyrG genes and a Streptomyces hygroscopicus bar gene.

The vector preferably contains an element(s) that permits integration of the vector into the host cell's genome or autonomous replication of the vector in the cell independent of the genome.

For integration into the host cell genome, the vector may rely on the polynucleotide's sequence encoding the polypeptide or any other element of the vector for integration into the genome by homologous or non-homologous recombination. Alternatively, the vector may contain additional polynucleotides for directing integration by homologous recombination into the genome of the host cell at a precise location(s) in the chromosome(s). To increase the likelihood of integration at a precise location, the integrational elements should contain a sufficient number of nucleic acids, such as 100 to 10,000 base pairs, 400 to 10,000 base pairs, and 800 to 10,000 base pairs, which have a high degree of sequence identity to the corresponding target sequence to enhance the probability of homologous recombination. The integrational elements may be any sequence that is homologous with the target sequence in the genome of the host cell. Furthermore, the integrational elements may be non-encoding or encoding polynucleotides. On the other hand, the vector may be integrated into the genome of the host cell by non-homologous recombination.

For autonomous replication, the vector may further comprise an origin of replication enabling the vector to replicate autonomously in the host cell in question. The origin of replication may be any plasmid replicator mediating autonomous replication that functions in a cell. The term “origin of replication” or “plasmid replicator” means a polynucleotide that enables a plasmid or vector to replicate in vivo.

Examples of origins of replication for use in a yeast host cell are the 2 micron origin of replication, ARS1, ARS4, the combination of ARS1 and CEN3, and the combination of ARS4 and CEN6.

Examples of origins of replication useful in a filamentous fungal cell are AMA1 and ANSI (Gems et al., 1991, Gene 98: 61-67; Cullen et al., 1987, Nucleic Acids Res. 15: 9163-9175; WO 00/24883). Isolation of the AMA1 gene and construction of plasmids or vectors comprising the gene can be accomplished according to the methods disclosed in WO 00/24883.

More than one copy of a polynucleotide of the present invention may be inserted into a host cell to increase production of a polypeptide. An increase in the copy number of the polynucleotide can be obtained by integrating at least one additional copy of the sequence into the host cell genome or by including an amplifiable selectable marker gene with the polynucleotide where cells containing amplified copies of the selectable marker gene, and thereby additional copies of the polynucleotide, can be selected for by cultivating the cells in the presence of the appropriate selectable agent.

The procedures used to ligate the elements described above to construct the recombinant expression vectors of the present invention are well known to one skilled in the art (see, e.g., Sambrook et al., 1989, supra).

Host Cells

A third aspect of the invention relates to a recombinant fungal host cell comprising one or more copies of a polynucleotide construct integrated in its genome, said construct comprising: a first polynucleotide encoding a selectable marker, wherein the first polynucleotide, a 5′ untranslated region thereof and/or a riboswitch operably linked therewith comprises a spliceosomal intron which has 5 nucleotides or less between its branch site and its acceptor site; and a second polynucleotide encoding a polypeptide of interest.

In a preferred embodiment of the third aspect, the fungal host cell comprises two or more copies of the polynucleotide construct integrated in its genome; even more preferably three or more copies and most preferably four or more copies of the polynucleotide construct integrated in its genome.

It is preferable that the integrative polynucleotide construct has been randomly integrated into the genome by non-homologous recombination or that it has been site-specifically integrated into the genome by homologous recombination; preferably into a gene encoding a nitrate reductase or nitrite reductase; more preferably into a gene required for gluconeogenesis; and most preferably into the niaD, niiA, acuN, acuK or acuM locus.

In a preferred embodiment the spliceosomal intron has 4 nucleotides or less between its branch site and its acceptor site; preferably 3 nucleotides or less; more preferably 2 nucleotides or less; even more preferably 1 nucleotide; still more preferably the spliceosomal intron has 0 nucleotides between its branch site and its acceptor site and most preferably the branch site and the acceptor site of spliceosomal intron overlap by at least one nucleotide.

In another preferred embodiment of the third aspect, the spliceosomal intron comprises a nucleotide sequence selected from the group of intron nucleotide sequences consisting of SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40.

It is preferred that the first polynucleotide of the integrative polynucleotide construct has a riboswitch operably linked therewith; preferably the riboswitch is derived from the thiA or nmtA genes in Aspergillus oryzae; even more preferably the riboswitch comprises the riboswitch of the thiA or nmtA genes in Aspergillus oryzae; most preferably the riboswitch consists of the riboswitch of the thiA or nmtA genes in Aspergillus oryzae.

Also preferred is that the first polynucleotide of the integrative polynucleotide construct encodes the selectable marker orotidine-5′ phosphate decarboxylase or PyrG.

Another preferred embodiment of the third aspect is that the polypeptide of interest encoded by the second polynucleotide of the integrative polynucleotide construct comprises an enzyme; preferably a hydrolase, isomerase, ligase, lyase, oxidoreductase, or transferase, e.g., an aminopeptidase, amylase, carbohydrase, carboxypeptidase, catalase, cellobiohydrolase, cellulase, chitinase, cutinase, cyclodextrin glycosyltransferase, deoxyribonuclease, endoglucanase, esterase, alpha-galactosidase, beta-galactosidase, glucoamylase, alpha-glucosidase, beta-glucosidase, invertase, laccase, lipase, mannosidase, mutanase, oxidase, pectinolytic enzyme, peroxidase, phytase, polyphenoloxidase, proteolytic enzyme, ribonuclease, transglutaminase, xylanase, or beta-xylosidase.

Preferably, the second polynucleotide of polynucleotide construct of the present invention is operably linked to one or more control sequences that direct the production of a polypeptide of the present invention. A construct or vector comprising a polynucleotide is introduced into a host cell so that the construct or vector is maintained as a chromosomal integrant or as a self-replicating extra-chromosomal vector as described earlier. The term “host cell” encompasses any progeny of a parent cell that is not identical to the parent cell due to mutations that occur during replication. The choice of a host cell will to a large extent depend upon the gene encoding the polypeptide and its source.

The host cell may be any cell useful in the recombinant production of a polypeptide of the present invention, e.g., a fungal cell.

“Fungi” as used herein includes the phyla Ascomycota, Basidiomycota, Chytridiomycota, and Zygomycota as well as the Oomycota and all mitosporic fungi (as defined by Hawksworth et al., In, Ainsworth and Bisby's Dictionary of The Fungi, 8th edition, 1995, CAB International, University Press, Cambridge, UK).

The fungal host cell may be a yeast cell. “Yeast” as used herein includes ascosporogenous yeast (Endomycetales), basidiosporogenous yeast, and yeast belonging to the Fungi lmperfecti (Blastomycetes). Since the classification of yeast may change in the future, for the purposes of this invention, yeast shall be defined as described in Biology and Activities of Yeast (Skinner, Passmore, and Davenport, editors, Soc. App. Bacteriol. Symposium Series No. 9, 1980).

The yeast host cell may be a Candida, Hansenula, Kluyveromyces, Pichia, Saccharomyces, Schizosaccharomyces, or Yarrowia cell, such as a Kluyveromyces lactis, Saccharomyces carlsbergensis, Saccharomyces cerevisiae, Saccharomyces diastaticus, Saccharomyces douglasii, Saccharomyces kluyveri, Saccharomyces norbensis, Saccharomyces oviformis, or Yarrowia lipolytica cell.

The fungal host cell may be a filamentous fungal cell. “Filamentous fungi” include all filamentous forms of the subdivision Eumycota and Oomycota (as defined by Hawksworth et al., 1995, supra). The filamentous fungi are generally characterized by a mycelial wall composed of chitin, cellulose, glucan, chitosan, mannan, and other complex polysaccharides. Vegetative growth is by hyphal elongation and carbon catabolism is obligately aerobic. In contrast, vegetative growth by yeasts such as Saccharomyces cerevisiae is by budding of a unicellular thallus and carbon catabolism may be fermentative.

The filamentous fungal host cell may be an Acremonium, Aspergillus, Aureobasidium, Bjerkandera, Ceriporiopsis, Chrysosporium, Coprinus, Coriolus, Cryptococcus, Filibasidium, Fusarium, Humicola, Magnaporthe, Mucor, Myceliophthora, Neocallimastix, Neurospora, Paecilomyces, Penicillium, Phanerochaete, Phlebia, Piromyces, Pleurotus, Schizophyllum, Talaromyces, Thermoascus, Thielavia, Tolypocladium, Trametes, or Trichoderma cell.

For example, the filamentous fungal host cell may be an Aspergillus awamori, Aspergillus foetidus, Aspergillus fumigatus, Aspergillus japonicus, Aspergillus nidulans, Aspergillus niger, Aspergillus oryzae, Bjerkandera adusta, Ceriporiopsis aneirina, Ceriporiopsis caregiea, Ceriporiopsis gilvescens, Ceriporiopsis pannocinta, Ceriporiopsis rivulosa, Ceriporiopsis subrufa, Ceriporiopsis subvermispora, Chrysosporium mops, Chrysosporium keratinophilum, Chrysosporium lucknowense, Chrysosporium merdarium, Chrysosporium pannicola, Chrysosporium queenslandicum, Chrysosporium tropicum, Chrysosporium zonatum, Coprinus cinereus, Coriolus hirsutus, Fusarium bactridioides, Fusarium cerealis, Fusarium crookwellense, Fusarium culmorum, Fusarium graminearum, Fusarium graminum, Fusarium heterosporum, Fusarium negundi, Fusarium oxysporum, Fusarium reticulatum, Fusarium roseum, Fusarium sambucinum, Fusarium sarcochroum, Fusarium sporotrichioides, Fusarium sulphureum, Fusarium torulosum, Fusarium trichothecioides, Fusarium venenatum, Humicola insolens, Humicola lanuginosa, Mucor miehei, Myceliophthora thermophila, Neurospora crassa, Penicillium purpurogenum, Phanerochaete chtysosporium, Phlebia radiata, Pleurotus etyngii, Thielavia terrestris, Trametes villosa, Trametes versicolor, Trichoderma harzianum, Trichoderma koningii, Trichoderma longibrachiatum, Trichoderma reesei, or Trichoderma viride cell.

Fungal cells may be transformed by a process involving protoplast formation, transformation of the protoplasts, and regeneration of the cell wall in a manner known per se. Suitable procedures for transformation of Aspergillus and Trichoderma host cells are described in EP 238023, Yelton et al., 1984, Proc. Natl. Acad. Sci. USA 81: 1470-1474, and Christensen et al., 1988, Bio/Technology 6: 1419-1422. Suitable methods for transforming Fusarium species are described by Malardier et al., 1989, Gene 78: 147-156, and WO 96/00787. Yeast may be transformed using the procedures described by Becker and Guarente, In Abelson, J. N. and Simon, M. I., editors, Guide to Yeast Genetics and Molecular Biology, Methods in Enzymology, Volume 194, pp 182-187, Academic Press, Inc., New York; Ito et al., 1983, J. Bacteriol. 153: 163; and Hinnen et al., 1978, Proc. Natl. Acad. Sci. USA 75: 1920.

Methods of Production

The present invention also relates to methods of producing a polypeptide of interest, comprising cultivating a recombinant fungal host cell according to the third aspect, and optionally recovering the polypeptide.

The host cells are cultivated in a nutrient medium suitable for production of the polypeptide using methods known in the art. For example, the cell may be cultivated by shake flask cultivation, or small-scale or large-scale fermentation (including continuous, batch, fed-batch, or solid state fermentations) in laboratory or industrial fermentors performed in a suitable medium and under conditions allowing the polypeptide to be expressed and/or isolated. The cultivation takes place in a suitable nutrient medium comprising carbon and nitrogen sources and inorganic salts, using procedures known in the art. Suitable media are available from commercial suppliers or may be prepared according to published compositions (e.g., in catalogues of the American Type Culture Collection). If the polypeptide is secreted into the nutrient medium, the polypeptide can be recovered directly from the medium. If the polypeptide is not secreted, it can be recovered from cell lysates.

The polypeptide may be detected using methods known in the art that are specific for the polypeptides. These detection methods include, but are not limited to, use of specific antibodies, formation of an enzyme product, or disappearance of an enzyme substrate. For example, an enzyme assay may be used to determine the activity of the polypeptide.

The polypeptide may be recovered using methods known in the art. For example, the polypeptide may be recovered from the nutrient medium by conventional procedures including, but not limited to, collection, centrifugation, filtration, extraction, spray-drying, evaporation, or precipitation.

The polypeptide may be purified by a variety of procedures known in the art including, but not limited to, chromatography (e.g., ion exchange, affinity, hydrophobic, chromatofocusing, and size exclusion), electrophoretic procedures (e.g., preparative isoelectric focusing), differential solubility (e.g., ammonium sulfate precipitation), SDS-PAGE, or extraction (see, e.g., Protein Purification, Janson and Ryden, editors, VCH Publishers, New York, 1989) to obtain substantially pure polypeptides.

In an alternative aspect, the polypeptide is not recovered, but rather a host cell of the present invention expressing the polypeptide is used as a source of the polypeptide.

EXAMPLES Materials and Methods

Growth of Strains on Solid Agar and in Liquid Medium

All strains have been grown on plates at 30° C. after transformation. When using liquid media, incubation was done at 30° C. with shaking at 180 rpm.

Lipase Assay

-   -   Dilution buffer:     -   50 mM Tris pH 7.5     -   10 mM CaCl2     -   0.1% Triton x-100     -   Substrate stock solution: 117 μl p-Nitrophenyl valerate (sigma         N4377) is diluted in 10 ml Methanol     -   Substrate: 10 ml dilution buffer is added 100 μl substrate stock         solution.         10 μl sample is added 1 ml substrate and product formation is         followed by measuring the absorbance at 405 nm.         Sucrose Medium         1M Sucrose         0.18 μM Na₂B₄O₇         2.3 μM CuSO₄         4.7 μM FeSO₄         4.7 μM MnSO₄         3.6 μM Na₂MoO₄         45 μM ZnSO₄         7 mM KCl         4.3 mM MgSO₄         11.2 mM KH₂PO₄         In-Fusion Cloning

In-Fusion Cloning was done using the In-Fusion cloning kit and manuals supplied by Clontech Laboratories, Inc.

Example 1: Making an Intron Library

The thiA promoter from Aspergillus oryzae and its 5′ untranslated region (UTR) riboswitch were amplified using the primers:

P722 (SEQ ID NO: 1):  atctcgagctcgcgaaagcttttcggtaaatacactatcacacac;  and P723 (SEQ ID NO: 2):  gagctcctcatggtggatccactagtgcatgccaagttgcaatgactat catctg.

The resulting 1338 bp fragment was In-Fusion cloned, using the Clontech In-Fusion kit, to the 7718 bp HindIII/BamHI fragment from the vector pCOIs207 (SEQ ID NO:21). The resulting vector was named pCOIs1086 and is shown in FIG. 1.

Plasmid pJaL1262 (full sequence shown in SEQ ID NO:22) was cut with SphI/AfeI, and the resulting 4840 bp fragment was purified.

Plasmid pCOIs1086 was cut with NheI, blunt-ended with Klenow and cut again with SphI and KasI. The resulting 4677 bp fragment was purified and ligated to the 4840 bp fragment from pJaL1262 and the resulting vector was named pCOIs1123.

The pyrG gene from Aspergillus oryzae was amplified from chromosomal DNA from Aspergillus oryzae A1560 using the primers:

P766 (SEQ ID NO: 3):  gtcattgcaacttggccaacatgtcttccaagtcgc;  and P767 (SEQ ID NO: 4):  accatgattacgccgcattagtgataccccactctaag.

The PCR product was In-Fusion cloned to the vector pCOIs1123 that had been linearized with SphI. The resulting vector was named pCOIs1124 and is shown schematically in FIG. 2.

An SOE-PCR product was made using pCOIs1124 as the template and the primers:

P722 (SEQ ID NO: 1):  atctcgagctcgcgaaagcttttcggtaaatacactatcacacac P769 (SEQ ID NO: 5):  ccgcggcaagttgcaatgactatcatctg P771 (SEQ ID NO: 6):  aattcgaaggcctcccatcaccatcaccatcactaag P503 (SEQ ID NO: 7):  acatgatcatataaccaattgccctcatccccatcctttaac

Plasmid pCOIs1118 shown in FIG. 3 (full sequence in SEQ ID NO:23) was cut with HindIII and SalI. The resulting 6762 bp fragment was ligated to the SOE-PCR product that had been cut with HindIII and SalI. The resulting plasmid was named pCOIs1126 and is shown in FIG. 4.

Eight different constructs were made that had different distances between the intron branch site “ctaac” (FIG. 5, shaded) and the intron acceptor site “tag” (FIG. 5, underscore) of the thiA riboswitch. The number of bases between the two sites were varied between 0-6 bp and one construct had the acceptor site overlapping with and forming a part of the branch site, resulting in the sequence “ctaacag”, where the branch site is “ctaac” and the acceptor site “cag”. The latter variant was named “Intron(−1)”. The others were named “Intron(0)”, “Intron(1)”, “Intron(2), “Intron(3)”, “Intron(4)”, “Intron(5)” and finally “Intron(6)” which is the wild type sequence (FIG. 5).

There are two stop-codons “TGATAG” in the same reading frame in the wildtype sequence Intron(6) (FIG. 5). It was decided to maintain at least two stop-codons in the variant constructs in the same reading frame as in the wildtype sequence in order to avoid any readthrough from unknown upstream gene(s). This was achieved by modifying the sequences slightly besides just removing one nucleotide from between the branch site and the acceptor site. So the sequences are not completely identical but this was not expected to have any significant effect on the splicing efficiencies of the constructs compared to the shortening of the distance between the branch and acceptor sites.

The constructs were made by PCR of the riboswitch in plasmid pCOIs1124 using the forward-primer: P782 (SEQ ID NO:8): ccttgatactctgtgagcgct; and

the following reverse primers, one per construct:

P783 (SEQ ID NO: 9): gagctcctcatggggccgcggcaatgactatcatctgttagccattccat caacagg P784 (SEQ ID NO: 10):  gagctcctcatggggccgcggcaatcaactatctcagttagccattccat caacagg P785 (SEQ ID NO: 11):  gagctcctcatggggccgcggcaatcagactattcagttagccattccat caacagg P786 (SEQ ID NO: 12):  gagctcctcatggggccgcggcaatcatcactatcagttagccattccat caacagg P787 (SEQ ID NO: 13):  gagctcctcatggggccgcggcaatcatcaactatcgttagccattccat caacagg P788 (SEQ ID NO: 14):  gagctcctcatggggccgcggcaatcatcagactatgttagccattccat caacagg P789 (SEQ ID NO: 15):  gagctcctcatggggccgcggcaatcatcatgactagttagccattccat caacagg P792 (SEQ ID NO: 16): gagctcctcatggggccgcggcaatgaggatcatctgttagccattccat caacagg

The PCR constructs were each In-Fusion cloned to the 7661 bp HindIII/SacII fragment from plasmid pCOIs1126. Table 1 below gives an overview of the primers used for creating the alternative introns (shown in FIG. 5) and the names of the resulting plasmids.

TABLE 1 Intron variant Reverse-primer Resulting plasmid Intron(6) SEQ ID NO: 9 pCOIs1135 Intron(5) SEQ ID NO: 10 pCOIs1136 Intron(4) SEQ ID NO: 11 pCOIs1137 Intron(3) SEQ ID NO: 12 pCOIs1138 Intron(2) SEQ ID NO: 13 pCOIs1139 Intron(1) SEQ ID NO: 14 pCOIs1140 Intron(0) SEQ ID NO: 15 pCOIs1141 Intron(−1) SEQ ID NO: 16 pCOIs1148

Example 2: Use of Intron Variants to Regulate Marker Expression

All the riboswitch intron variants were fused to the lipase reporter gene of pCOIs1126. The resulting constructs were integrated at the niaD locus in one copy in an Aspergillus oryzae strain having a partially deleted niaD gene which was repaired by the individual in-coming constructs, thereby enabling growth on nitrate as the sole nitrogen source.

Transformants were grown in Sucrose Medium for three days and lipase activity was measured on the supernatant as detailed above. The lipase activities found are summarized in table 2 below:

TABLE 2 Intron variant Relative reporter activity % of wild type Intron(6) 0.244 100 Intron(5) 0.231 95 Intron(4) 0.178 73 Intron(3) 0.066 27 Intron(2) 0.018 7 Intron(1) 0.010 4 Intron(0) 0.006 2.5 Intron(−1) 0.000 0

From these results, we conclude that it is possible to make a series of intron-containing 5-UTRs with different intron splicing efficiencies providing a range of expression levels of the downstream operably linked gene. When the splicing efficiency is lowered, a proportion of the expressed mRNAs will contain the intron sequence, resulting in translation initiation and immediate translation termination within the intron, preventing translation of the downstream gene product—as demonstrated with the lipase activity in table 2.

Example 3: Intron Riboswitch Variants are Repressible by Thiamine

Four of the thiA riboswitch intron variant strains: (6), (3), (0) and (−1), were grown in sucrose medium for 48 hours without or with a supplement of thiamine (10 uM of ThiamineHCl), respectively. Samples were analysed for lipase activity as already shown—the results are in table 3 below.

We were unable to determine (“UD”) any activity from Intron(0) and Intron(−1), but the wildtype sequence in Intron(6) was actually repressed by almost a factor of 100 with addition of thiamine, as expected, and Intron(3) was repressed by almost a factor of 10 on top of the reduction due to the shortened branch and acceptor site distance in that variant.

TABLE 3 Intron variant −ThiamineHCl +ThiamineHCl Intron(6) 100 1.6 Intron(3) 8.3 1.4 Intron(0) UD UD Intron(−1) UD UD

Example 4: Use of Intron Variants to Select for Transformed Strains with a High Copy Number of an Integrated Gene

This example illustrates the use of the intron variants and their thiamine repression to control expression of a selectable marker (rather than an expression reporter) in order to obtain transformants with a high gene copy number of the marker gene. The marker gene is linked with an expression cassette comprising a gene of interest which we would like to express at a high level; the expression cassette will be duplicated along with the marker gene, thus increase its copy number as well.

The wild type thiA riboswitch, Intron(6), and three intron variants, Intron(3), Intron(0) and Intron(−1) were cloned in front of the pyrG gene from Aspergillus oryzae. The constructs were fused to a lipase expression cassette. Expression from the cassette would be used as a benchmark for the gene copy number.

A plasmid pCOIs1130 was made by doing SOE-PCR on pCOIs1124 (FIG. 2) using the primerpairs P722/P769 and P775/767:

P722 (SEQ ID NO: 1):  atctcgagctcgcgaaagcttttcggtaaatacactatcacacac P769 (SEQ ID NO: 5):  ccgcggcaagttgcaatgactatcatctg P775 (SEQ ID NO: 17):  gatagtcattgcaacttgccgcggcaccatgtcttccaagtcgcaattgac P767 (SEQ ID NO: 18):  accatgattacgccgcattagtgataccccactctaag

An SOE-PCR was then made using the primerpair P722/P767 and the resulting fragment was cut with HindIII/BsiWI and ligated to a 8389 bp HindIII/BsiWI fragment from pCOIs1124. The resulting plasmid was named pCOIs1130.

Construction of Four Intron Variant Constructs Fused to pyrG

The plasmid pCOIs1130 was cut with HindIII/SacII and the 9406 bp fragment was purified.

The plasmids pCOIs1135, pCOIs1138, pCOIs1141 and pCOIs1148 (see table 1) were all cut with HindIII/SacII and the 1286 bp fragment was ligated to the 9406 bp fragment from pCOIs1130. The resulting plasmids were named according to table 4 below.

TABLE 4 Intron variant Plasmid Intron(6) pCOIs1172 Intron(3) pCOIs1173 Intron(0) pCOIs1174 Intron(−1) pCOIs1175

The four plasmids pCOIs1172, pCOIs1173, pCOIs1174 and pCOIs1175 were all individually transformed in an A. oryzae strain that had the pyrG deleted and had a partial niaD gene that could be repaired by integrating the plasmid at the niaD locus. The transformation mixtures were spread on plates containing three different media:

A) Sucrose Medium+10 mM NaNO₃+20 mM Uridine

B) Sucrose Medium+10 mM NaNO₃

C) Sucrose Medium+10 mM NaNO₃+10 μM ThiamineHCl

Transformants were counted on the different plates and the results are shown in table 5 below.

TABLE 5 Intron variant Medium A Medium B Medium C Intron(6) 100 100 100 Intron(3) 100 79 63 Intron(0) 100 33 total 22 total 6 large 4 large Intron(−1) 100 24 total 23 total 6 large 7 large

On medium B and medium C, where expression of the pyrG is required, we found transformants that grew differently. We categorized some of the transformants as being large which were those transformants that were healthy and grew with the same speed as a wild type strain. The other transformants grew much more slowly.

The number of transformants on Medium C is lower than on Medium B for Intron(3) and Intron(0), indicating that there is a thiamine effect which represses expression of the pyrG gene. We conclude that when using Intron(3), Intron(0) and Intron(−1) in front of the pyrG marker, more than one copy of the pyrG gene is needed in order for the cell to grow.

We saw that the so-called large colonies obtained with the Intron(0) and Intron(−1) constructs contain a very high copy-number of the expression construct. This was determined by Southern analysis digesting chromosomal DNA with AatII and AfeI and then using part of the niaD integration fragment as the probe (see FIG. 6). In FIG. 6, one can see that the background strain has one fragment binding the probe, whereas the strain transformed with pCOIs1175 show binding of the probe to fragments containing sequences flanking the insertion site as well as the whole expression construct, which show a band with a very high intensity; indicating many copies of the expression construct.

Example 5: Riboswitches in Tandem Provides a Broader Range of Thiamine Repression

Multiple thiA riboswitches in front of a specific gene will increase the likelihood that the messenger-RNA will enter the cytoplasm with one of the riboswitches intact, if the thiamine concentration within the cell allows for some degree of occupation of the TPP binding site. Thus, at a high concentration of thiamine, two or more thiA riboswitches should facilitate increased thiamine repression of an operably linked downstream coding sequence.

A thiA riboswitch was constructed synthetically in one or two copies and placed after the thiA riboswitch and in front of the lipase reporter gene of pCOIs1126 (FIG. 4), resulting in the plasmids pCOIs1142 (2 riboswitches) and pCOIs1143 (3 riboswitches).

The 5′-end of the riboswitches was SEQ ID NO:19: ccgctcccacacaattctct

The 3′-end of the riboswitches was SEQ ID NO:20: tcattgcaacttg

The constructs were integrated in A. oryzae at the niaD locus in one copy resulting in the strains shown in table 7.

TABLE 7 Strain Plasmid Riboswitch copies Reporter activity Ribo1 pCOIs1126 1 100%  Ribo2 pCOIs1142 2 48% Ribo3 pCOIs1143 3 32%

The three strains in Table 7 were grown in sucrose medium without added thiamine for 48 hours at 30° C. Samples were taken and analysed for lipase reporter activity, as mentioned above.

The activity from the Ribo2 strain in table 7 was only 48% compared to the Ribo1 strain and the Ribo3 strain only produced 32% of the activity found in the Ribo1 strain. These results demonstrate that the presence of two or more thiA riboswitches in front of a given gene will lower its expression accordingly.

We also performed a similar experiment with addition of 1 uM of thiamine to the medium, but the expression level from the Ribo2 and Ribo3 strains were below the detection level of our lipase assay. This indicates a high degree of repression using these tandem constructs as well as a broader range of repression than when using only one riboswitch.

Example 6: Riboswitch Introns Inside an Open Reading Frame Overcomes any Problem with Ribosome Reinitiation

We have observed that with any riboswitch construct we put in front of the pyrG marker and with a concentration of thiamine that we expected to give full repression, we could still always obtain a few transformants. We believe this is either due to ribosome read-through of the riboswitch intron without initiating at any of the ATG codons present in the intron or from reinitiation events at the pyrG start codon.

To bring expression of the pyrG marker to even lower levels, the riboswitch intron can be moved inside the pyrG open reading frame. This will prevent formation of a functional pyrG expression product if the intron has not been spliced out.

Example 7: Making a Synthetic Intron Library

A synthetic intron was constructed from consensus sequences of the donor site, branch site and acceptor site of the second intron in the riboswitch from the thiA gene from Aspergillus oryzae. The synthetic intron contains two very efficient translation start sites both having a very good kozak sequence and being in different frames. These two open reading frames within the intron are terminated before the intron branch site. Moreover, the intron contains six other translation start sites with less efficient kozaks, thus contributing less to translational initiation of ribosomes having missed upstream translation start sites. The modified intron was fused to the sequence “tgtacattgattaattgacaccATG” (SEQ ID NO:24), which contains stop codons in all three reading frames, which will block ribosomes translating from translation start sites within the intron. Moreover SEQ ID NO:24 contains at the 3′-end the kozak and translation initiation codon for the lipase reporter gene used in the example.

Eight different variants of the synthetic intron were made by changing the distance between the branch site “ctaac” sequence and the splice acceptor site “tag”. In the eighth sequence, Intron(−1), the splice acceptor site had the first base of the triplet “tag” deleted so the branch site and acceptor site were fused into the sequence “ctaacag”, where the acceptor site is part of the branch site. See Table 8 for an overview of the eight intron variants.

All eight intron variants were cloned in the reporter construct present in pCOIs1126 between the splice branch site and the translational start codon of the lipase reporter gene. The resulting constructs were integrated individually at the niaD locus in one copy in an Aspergillus oryzae strain having a partially deleted niaD gene, which was repaired by the individual in-coming construct, thereby enabling growth on nitrate as the sole nitrogen source. The individual construct was verified to be present in only one copy in order to exclude a copy number effect on the results.

TABLE 8 Splice site variant Strain SEQ ID NO ctaacagttgatagtgtacattgattaattgacaccATG Intron(6) SEQ ID NO: 25 ctaacgttgatagtgtacattgattaattgacaccATG Intron(5) SEQ ID NO: 26 ctaacttgatagtgtacattgattaattgacaccATG Intron(4) SEQ ID NO: 27 ctaactgatagtgtacattgattaattgacaccATG Intron(3) SEQ ID NO: 28 ctaacgatagtgtacattgattaattgacaccATG Intron(2) SEQ ID NO: 29 ctaacatagtgtacattgattaattgacaccATG Intron(1) SEQ ID NO: 30 ctaactagtgtacattgattaattgacaccATG Intron(0) SEQ ID NO: 31 ctaacagtgtacattgattaattgacaccATG Intron(-1) SEQ ID NO: 32

Because of the requirement of some spacing between the branch site and the splice acceptor site for efficient splicing, the constructs with the less spacing had less expression of the reporter gene, because translation will be initiated at the translation start sites within the non-spliced intron and terminated before reaching the lipase reporter gene. Varying spacing between the branch site and the acceptor is thereby an efficient way of varying gene expression. In this example, where the thiA promoter and thiA riboswitch have been used, additional variation can even be obtained by varying the exogenous supply of thiamine. However, the endogenous thiamine concentration will vary in different growth phases influencing thiamine repression, whereas the efficiency of splicing is expected to be constant. Thus, a constant regulation of gene expression can be obtained by using splice site variants instead of using inducible or repressible promoters. This can be important when a constant and exact level of gene expression is wanted. 

The invention claimed is:
 1. A polynucleotide construct suitable for transformation into a fungal host cell and integration into the genome of said cell, said construct comprising: (a) a first polynucleotide encoding a selectable marker, wherein the first polynucleotide, a 5′ untranslated region thereof and/or a riboswitch operably linked therewith comprises a spliceosomal intron which has 5 or less nucleotides between its branch site and its acceptor site; and (b) a second polynucleotide encoding a polypeptide of interest.
 2. The polynucleotide construct of claim 1, wherein the fungal host cell is a filamentous fungal host cell.
 3. The polynucleotide construct of claim 1, wherein the fungal host cell is a yeast host cell.
 4. The polynucleotide construct of claim 1, which further comprises a gene encoding a functional nitrate reductase, a gene encoding a functional nitrite reductase, or a gene encoding a functional enolase.
 5. The polynucleotide construct of claim 1, which is flanked on one or both side(s) by a homology box of sufficient size and sequence homology to a specific locus in the fungal host cell genome to enable site-specific integration of the integrative polynucleotide construct into said genome by homologous recombination after transformation.
 6. The polynucleotide construct of claim 1, wherein the spliceosomal intron has 4 nucleotides or less between its branch site and its acceptor site.
 7. The polynucleotide construct of claim 1, wherein the spliceosomal intron comprises a nucleotide sequence selected from the group of intron nucleotide sequences consisting of SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO:
 40. 8. The polynucleotide construct of claim 1, wherein the first polynucleotide has a riboswitch operably linked therewith.
 9. The polynucleotide construct of claim 1, wherein the first polynucleotide of the integrative polynucleotide construct encodes the selectable marker orotidine-5′-phosphate decarboxylase or PyrG.
 10. The polynucleotide construct of claim 1, wherein the polypeptide of interest encoded by the second polynucleotide of the integrative polynucleotide construct comprises an enzyme.
 11. A recombinant fungal host cell comprising one or more copies of the polynucleotide construct of claim 1 integrated in its genome.
 12. The recombinant fungal host cell of claim 11 which comprises two or more copies of the polynucleotide construct integrated in its genome.
 13. The recombinant fungal host cell of claim 11 which is a filamentous fungal host cell.
 14. The recombinant fungal host cell of claim 11 which is a yeast host cell.
 15. The recombinant fungal host cell of claim 11, wherein the integrative polynucleotide construct is randomly integrated into the genome by non-homologous recombination.
 16. The recombinant fungal host cell of claim 11, wherein the integrative polynucleotide construct is site-specifically integrated into the genome by homologous recombination.
 17. The recombinant fungal host cell of claim 11, wherein the spliceosomal intron has 4 nucleotides or less between its branch site and its acceptor site.
 18. The recombinant fungal host cell of claim 11, wherein the spliceosomal intron comprises a nucleotide sequence selected from the group of intron nucleotide sequences consisting of SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO:
 40. 19. The recombinant fungal host cell of claim 11, wherein the first polynucleotide of the integrative polynucleotide construct has a riboswitch operably linked therewith.
 20. The recombinant fungal host cell of claim 11, wherein the first polynucleotide of the integrative polynucleotide construct encodes the selectable marker orotidine-5′-phosphate decarboxylase or PyrG.
 21. The recombinant fungal host cell of claim 11, wherein the polypeptide of interest encoded by the second polynucleotide of the integrative polynucleotide construct comprises an enzyme.
 22. A method of producing a polypeptide of interest, comprising: (a) cultivating the recombinant fungal host cell of claim 11; and optionally (b) recovering the polypeptide.
 23. The polynucleotide construct of claim 2, wherein the filamentous fungal host cell is an Acremonium, Aspergillus, Aureobasidium, Bjerkandera, Ceriporiopsis, Chrysosporium, Coprinus, Coriolus, Cryptococcus, Filibasidium, Fusarium, Humicola, Magnaporthe, Mucor, Myceliophthora, Neocallimastix, Neurospora, Paecilomyces, Penicillium, Phanerochaete, Phiebia, Piromyces, Pleurotus, Schizophyllum, Talaromyces, Thermoascus, Thielavia, Tolypocladium, Trametes, or Trichoderma cell.
 24. The polynucleotide construct of claim 3, wherein the yeast host cell is a Candida, Hansenula, Kluyveromyces, Pichia, Saccharomyces, Schizosaccharomyces, or Yarrowia cell.
 25. The polynucleotide construct of claim 4, wherein the gene encoding a functional nitrate reductase is niaD.
 26. The polynucleotide construct of claim 4, wherein the gene encoding a functional nitrite reductase is niiA.
 27. The polynucleotide construct of claim 4, wherein the gene encoding a functional enolase is acuN.
 28. The polynucleotide construct of claim 6, wherein the spliceosomal intron has 3 nucleotides or less between its branch site and its acceptor site.
 29. The polynucleotide construct of claim 6, wherein the spliceosomal intron has 2 nucleotides or less between its branch site and its acceptor site.
 30. The polynucleotide construct of claim 6, wherein the spliceosomal intron has 1 nucleotide between its branch site and its acceptor site.
 31. The polynucleotide construct of claim 6, wherein the branch site and the acceptor site of the spliceosomal intron overlap by at least one nucleotide.
 32. The polynucleotide construct of claim 8, wherein the riboswitch is derived from the thiA gene in Aspergillus oryzae.
 33. The polynucleotide construct of claim 8, wherein the riboswitch is derived from the nmtA gene in Aspergillus oryzae.
 34. The recombinant fungal host cell of claim 11 which comprises three or more copies of the polynucleotide construct integrated in its genome.
 35. The recombinant fungal host cell of claim 11 which comprises four or more copies of the polynucleotide construct integrated in its genome.
 36. The recombinant fungal host cell of claim 13, wherein the filamentous fungal host cell is an Acremonium, Aspergillus, Aureobasidium, Bjerkandera, Ceriporiopsis, Chrysosporium, Coprinus, Coriolus, Cryptococcus, Filibasidium, Fusarium, Humicola, Magnaporthe, Mucor, Myceliophthora, Neocallimastix, Neurospora, Paecilomyces, Penicillium, Phanerochaete, Phiebia, Piromyces, Pleurotus, Schizophyllum, Talaromyces, Thermoascus, Thielavia, Tolypocladium, Trametes, or Trichoderma cell.
 37. The recombinant fungal host cell of claim 14, wherein the yeast host cell is a Candida, Hansenula, Kluyveromyces, Pichia, Saccharomyces, Schizosaccharomyces, or Yarrowia cell.
 38. The recombinant fungal host cell of claim 16, wherein the integrative polynucleotide construct is site-specifically integrated into the genome by homologous recombination into a gene encoding a nitrate reductase, a gene encoding a nitrite reductase, or a gene encoding an enolase.
 39. The recombinant fungal host cell of claim 38, wherein the nitrate reductase gene is niiD.
 40. The recombinant fungal host cell of claim 38, wherein the nitrite reductase gene is niiA.
 41. The recombinant fungal host cell of claim 38, wherein the enolase gene is acuN, acuK, or acuM.
 42. The recombinant fungal host cell of claim 17, wherein the spliceosomal intron has 3 nucleotides or less between its branch site and its acceptor site.
 43. The recombinant fungal host cell of claim 17, wherein the spliceosomal intron has 2 nucleotides or less between its branch site and its acceptor site.
 44. The recombinant fungal host cell of claim 17, wherein the spliceosomal intron has 1 nucleotide between its branch site and its acceptor site.
 45. The recombinant fungal host cell of claim 17, wherein the branch site and the acceptor site of the spliceosomal intron overlap by at least one nucleotide.
 46. The recombinant fungal host cell of claim 19, wherein the riboswitch is derived from the thiA gene in Aspergillus oryzae.
 47. The recombinant fungal host cell of claim 19, wherein the riboswitch is derived from the nmtA gene in Aspergillus oryzae. 